1. Field of the Invention
The present invention relates generally to plastic containers; particularly to plastic containers designed to hold liquids under pressure.
2. Background Art
Bottle bases comprising the standard champagne dome have long been employed with glass bottles and consumers have become accustomed to such base configurations. The standard champagne dome, as is well known in the art, is an arcuate dome that extends inwardly of the container from the chime portion of the container base. The champagne dome distributes forces exerted thereon by the internal pressure of the bottle. The standard champagne dome shape has evolved into various dome shapes in attempts to better withstand these forces. The integrity of the champagne dome becomes of greater importance when the bottle is intended to contain a product under pressured by, for example, carbonation. While the use of a champagne dome is desirable, application thereof to plastic containers has provided difficulties in the plastic container industry.
For reasons of efficiency and to lower production costs, the plastic container industry has embraced the conventional technique of blow molding plastic containers from plastic preforms. Furthermore, the industry often uses polyethylene terephthalate ("PET") or polypropylene ("PP") to construct plastic containers because of, among other reasons, the ability to reclaim and recycle containers constructed therefrom. A barrier layer constructed, for example, from ethylene vinyl alcohol ("EVOH"), is often employed with the PET or PP to inhibit the migration of gases such as oxygen and carbon dioxide as well as moisture into or out of, the container. In order to provide a plastic container constructed of PET or PP with desirable strength and clarity characteristics, it is desirable to impart bi-axial stretching to the material because the strength of any portion of a container blow molded from these materials directly correlates to the degree of bi-axial stretching experienced by that portion during the blow molding process. Thus, because the bi-axial stretching experienced at the base of a plastic container is relatively small in comparison to that experienced at other portions of the container, a relative decrease in material strength results at the base. Moreover, the formation of the preforms used for blow molding are typically formed by injection molding and leave an injection gate nib at the portion of the preform which corresponds to the plastic container base once the preform has been blown. The presence of the nib at the base has also been shown to result in decreased base strength. For all of the above reasons, the strength of the base of a plastic container is of specific concern to the plastic container industry. As with glass containers, the strength of a plastic container base is of special importance when employed to contain pressurized products such as a carbonated liquid including, for example, beer and soda.
The plastic container industry has found the standard champagne dome base to be an unacceptable configuration for blow molding of plastic bottles because, for example, the standard champagne dome has been found to be susceptible to inversion when constructed from plastic. Therefore, plastic container manufacturers have turned to alternative base configurations.
However, these alternative configurations typically comprise intricate base configurations having deficiencies rendering them unacceptable as well. For example, prior base configurations have replaced the standard chime with a plurality of feet segmented by ribs extending upward from the base. This configuration required more material than configurations not employing feet and ribs and thereby increased the overall cost of the configuration. Accordingly, in an attempt to reduce material consumption, the outer diameter of the base of this configuration was drastically reduced inward toward the longitudinal axis of the bottle and the cylindrical wall was gradually curved inward to the reduced outer diameter of the base. This reduced base configuration intruded substantially into a portion of the bottle that was traditionally reserved for the cylindrical wall of the body from which the base extends. Because the outer diameter of the lower most portion of the bottle was greatly reduced, the diameter of the contact ring upon which the bottle would be expected to stand and, therefore, the ability of a bottle comprising this base configuration to remain erect, was drastically reduced.
Importantly, the feet, ribs and reduced outer diameter of this base configuration provided an appearance to consumers which differed substantially from traditional configurations such as those commonly found in glass containers and to which consumers had become accustomed. Indeed, customer expectations call for a base configuration that does not contain feet. The beer industry provides one example of these customer expectations. It is desirable at certain times (e.g. sporting events) to provide consumers with beer contained in plastic bottles rather than glass bottles which can cause personal injuries; especially when broken. However, consumers have become accustomed to certain bottle shapes in which they expect beer to be contained. Variation from these shapes may result in loss of sales or good will. Accordingly, the intricate configurations of prior art plastic bottle bases are not applicable to all products for which a plastic container is desired and, specifically, not acceptable as containers for beer.
A standard practice used to blow mold plastic containers is to slightly increase the thickness of a majority of the base relative to the thickness of the remainder of the bottle. Preforms used to construct a bottle having such a base are known and do not require complex configurations. However, other attempts to blow mold an acceptable plastic bottle have placed material concentrations in specific predetermined areas of the base to increase the amount of stress that could be withstood without failing. One such base configuration used a stepped base to increase the thickness of the dome to a thickness that is substantially thicker than the dome of a standard base. This configuration increased material consumption and the difficulty of constructing the preforms. Because plastic containers are usually produced in extremely high volume, economies of scale made these configurations prohibitive.
Another example of using material concentrations teaches a plastic bottle base of the champagne base variety having a reinforced hoop to deter the champagne dome from inverting due to internal pressure of the bottle. However, an intricate preform was required to direct material concentrations to the necessary areas of the base to form the reinforced hoop. Increased costs and production difficulties resulted in the manufacture of these preforms as well.